Use C locale when reading ReplayGain tag

Use C locale when reading ReplayGain tag

When a locale is in effect that does not use the point as the decimal
mark (e.g., sv_SE or de_DE, which use a comma) and a ReplayGain tag is
read for --apply-replaygain-which-is-not-lossless, the gain value was
misinterpreted (e.g., "-2.29" truncated to "-2"). This is fixed by
resetting the locale to "C" temporarily, based on Josh Coalson's fix
of the dual case (writing ReplayGain tag) in commit cda02d3.

Patch by hhaamu@gmail.com, taken from the Debian patch tracker for
flac 1.2.1-6 (13_replaygain_c_locale.patch).

http://sourceforge.net/p/flac/bugs/380/

Signed-off-by: Erik de Castro Lopo <erikd@mega-nerd.com>