Added functions to convert GError messages (which are in UTF-8 charset)

Added functions to convert GError messages (which are in UTF-8 charset)
to terminal charset.

src/strutil.h: added conv_gerror_message member to str_class structure.
Added str_conv_gerror_message function declaration.

src/strutil.c (str_conv_gerror_message): new function to convert GError
message to terminal charset.

src/strutil8bit.c (str_8bit_conv_gerror_message): new function to convert GError
message to terminal charset for 8-bit locales.

src/strutilascii.c (str_ascii_conv_gerror_message): new function to convert GError
message to terminal charset for 7-bit locales.

src/strutilutf8.c (str_utf8_conv_gerror_message): new function to convert GError
message to terminal charset for utf-8 locales.