[multiple] spelling: github action check-spelling

[multiple] codespell: correct spelling in comments

[multiple] use buffer_append_char()

[core] feature flag to allow Range w/ HTTP/1.0

server.feature-flags += ( "http10.range" => "enable" )

The Range request header is HTTP/1.1, not HTTP/1.0.

Intermediate HTTP/1.0 proxies might mishan

[core] feature flag to allow Range w/ HTTP/1.0

server.feature-flags += ( "http10.range" => "enable" )

The Range request header is HTTP/1.1, not HTTP/1.0.

Intermediate HTTP/1.0 proxies might mishandle or incorrectly cache
responses to HTTP/1.1 Range requests, so the default in lighttpd is
to ignore Range requests sent with HTTP/1.0.

This feature flag changes the default if an admin desires to support
dumb HTTP/1.0 clients that might (incorrectly) send Range requests
with HTTP/1.0. Those client really ought to grow HTTP/1.1 support:
add support to receive HTTP/1.1 Transfer-Encoding: chunked responses,
and then those client may safely send HTTP/1.1 Range requests
(and in many cases, also Connection: close).

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[core] replace strncasecmp w/ buffer_eq_icase_ssn

replace strncasecmp() w/ buffer_clen() and buffer_eq_icase_ssn()
(portability; remove use of alt sys-strings.h portability header)

[multiple] reduce redundant NULL buffer checks

This commit is a large set of code changes and results in removal of
hundreds, perhaps thousands, of CPU instructions, a portion of which
are on hot co

[multiple] reduce redundant NULL buffer checks

This commit is a large set of code changes and results in removal of
hundreds, perhaps thousands, of CPU instructions, a portion of which
are on hot code paths.

Most (buffer *) used by lighttpd are not NULL, especially since buffers
were inlined into numerous larger structs such as request_st and chunk.

In the small number of instances where that is not the case, a NULL
check is often performed earlier in a function where that buffer is
later used with a buffer_* func. In the handful of cases that remained,
a NULL check was added, e.g. with r->http_host and r->conf.server_tag.

- check for empty strings at config time and set value to NULL if blank
string will be ignored at runtime; at runtime, simple pointer check
for NULL can be used to check for a value that has been set and is not
blank ("")
- use buffer_is_blank() instead of buffer_string_is_empty(),
and use buffer_is_unset() instead of buffer_is_empty(),
where buffer is known not to be NULL so that NULL check can be skipped
- use buffer_clen() instead of buffer_string_length() when buffer is
known not to be NULL (to avoid NULL check at runtime)
- use buffer_truncate() instead of buffer_string_set_length() to
truncate string, and use buffer_extend() to extend

Examples where buffer known not to be NULL:
- cpv->v.b from config_plugin_values_init is not NULL if T_CONFIG_BOOL
(though we might set it to NULL if buffer_is_blank(cpv->v.b))
- address of buffer is arg (&foo)
(compiler optimizer detects this in most, but not all, cases)
- buffer is checked for NULL earlier in func
- buffer is accessed in same scope without a NULL check (e.g. b->ptr)

internal behavior change:
callers must not pass a NULL buffer to some funcs.
- buffer_init_buffer() requires non-null args
- buffer_copy_buffer() requires non-null args
- buffer_append_string_buffer() requires non-null args
- buffer_string_space() requires non-null arg

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[multiple] use buffer_append_* aggregates

reduces the number of round-trips into some frequently-called routines

[multiple] http_header APIs to reduce str copies

[core] quiet coverity warning

[core] http_range.[ch] RFC 7233 Range handling

(import from one of my development branches from 2015)