Force the use of '.' as decimal separator.

This solves issues occurring with a different decimal operator and
keeps the command line interface consistent for all locales .
E.g. `zfs set quota=0.5T`

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Felix Neumärker <xdch47@posteo.de>
Closes #10878

cmd/mount_zfs/mount_zfs.c

@@ -182,6 +182,7 @@ main(int argc, char **argv)
 	int error, c;
 
 	(void) setlocale(LC_ALL, "");
+	(void) setlocale(LC_NUMERIC, "C");
 	(void) textdomain(TEXT_DOMAIN);
 
 	opterr = 0;

cmd/zfs/zfs_main.c

@@ -8468,6 +8468,7 @@ main(int argc, char **argv)
 	char **newargv;
 
 	(void) setlocale(LC_ALL, "");
+	(void) setlocale(LC_NUMERIC, "C");
 	(void) textdomain(TEXT_DOMAIN);
 
 	opterr = 0;

cmd/zpool/zpool_main.c

@@ -10229,6 +10229,7 @@ main(int argc, char **argv)
 	char **newargv;
 
 	(void) setlocale(LC_ALL, "");
+	(void) setlocale(LC_NUMERIC, "C");
 	(void) textdomain(TEXT_DOMAIN);
 	srand(time(NULL));