Strip down file util functions

Drop unused funcs in gutils/fsys.c
inc/fileutil.h is a partial list of inc/gfile.h, so drop it
Some funcs are just dummy wrapper of standard C or glib functions
Fix free-before-use (why it doesn't occur earlier?)
GFileWriteAll → g_file_set_contents
u_GFileGetHomeDir → GFileGetHomeDir
GFileMkDir → g_mkdir
Cosmetics
GFileIsAbsolute → g_path_is_absolute
Fix g_mkdir calls
Rewrite _GPSDraw_InitFonts() to use glib util funcs
GFileBuildName() orphaned
GFileIsDir → g_file_test
GFileExists → g_file_test
Merge branch 'slimming-4'
Fix compile due to GFileExists elimination