util: New flag UNQUOTE_UNESCAPE_RELAX for unquote_first_word

util: New flag UNQUOTE_UNESCAPE_RELAX for unquote_first_word

The new flag UNQUOTE_UNESCAPE_RELAX preserves unrecognized escape
sequences verbatim in unquote_first_word, either when it's a trailing
backslash (similar to UNQUOTE_RELAX, but in this case keep the extra
backslash in the output) or in the middle of a sequence string.

Add unit test cases to ensure the new flag works as expected and to
prevent regressions from being introduced.

Tested with a follow up commit converting config_parse_exec() to start
using unquote_first_word, in which case this flags makes it possible to
preserve unrecognized escape sequences.

Relevant bug: https://bugs.freedesktop.org/show_bug.cgi?id=90794