ash: factor out ASH_HELP config option

It used to be aliased to !FEATURE_SH_EXTRA_QUIET for ash,
while hush had it separate from FEATURE_SH_EXTRA_QUIET.

Bring ash in line with hush.

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
ash: fix off-by-one in "jobs %4" handling. closes 7310

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
mdev: treat zero-length /dev/mdev.seq the same as "\n" one. Closes 7334

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>