The `parse_section_header_ext` name suggests that it as an extended
function for parsing the section header.  It is not.  Rename it to
`parse_subsection_header` to better reflect its true mission.

When we reach a whitespace after a section name, we assume that what
will follow will be a quoted subsection name.  Pass the current position
of the line being parsed to the subsection parser, so that it can
validate that subsequent characters are additional whitespace or a
single quote.

Previously we would begin parsing after the section name, looking for
the first quotation mark.  This allows invalid characters to embed
themselves between the end of the section name and the first quotation
mark, eg `[section foo "subsection"]`, which is illegal.

When we don't specify a particular column, don't write it in the error
message.  (column "0" is unhelpful.)

Update the configuration parsing error messages to be lower-cased for
consistency with the rest of the library.

Loosen restriction on wildcard "*" refspecs

Use PCRE for our fallback regex engine when regcomp_l is unavailable

Since libssh2 doesn't read host configuration from the config file,
this callback can be used to hand over URL resolving to the client
without touching the SSH implementation itself.

This avoids any misunderstanding with the REGEX keyword in cmake.

Skip UTF8 BOM in ignore files