option_parser.c: fix load_ini_file()

The bit of the parser that dealt with quoting and comments was broken.  I
have fixed it by having it crawl (-ish; using strbrk) the string for
characters that it treats specially, so that multiple special characters
are all parsed correctly.
option_parser.c: optimize calls to g_memmove to aviod

It was copying A to B, the B to C; we can be clever and copy A directly to
C.  Of course, this means it is less readable/obvious what it is doing.