Allow setting custom glyphs for progress bar

This allows using custom unicode glyphs to render progress bars.

Requires a new setting GlyphBar =  in pacman.conf (Use Fira Code to see these glyphs)
The setting is parsed but ignored if ILoveCandy is also set.
The 6 glyphs in the string are [emptybar_beg, emptybar_mid, emptybar_end, fullbar_beg, fullbar_mid, fullbar_end] in that order.

Technically, any unicode chars can be used, even emoji, but the emoji's double-width will cause chaos, so don't 🤷

Try these:
GlyphBar = ╠═╣┣━┫
GlyphBar = पगचदटड

pacman-conf.c also required a small refactor to be able to handle multibyte characters.
Additionally, makepkg.sh's write_buildinfo() which always defaulted to LC_ALL=C needed a change to not break when using GlyphBar. It now uses LC_ALL=C.UTF-8 if the user's locale's encoding is UTF-8, and falls back to LC_ALL=C otherwise.

Feedback is welcome! Let me know if anything needs to be changed, or is missing.

Screenshot: