devfs: fix locking and ignore undocumented "fsdev:\n" configuration signature

devfs: fix locking and ignore undocumented "fsdev:\n" configuration signature

The confstr was shared between readers so seprintconf() could
write concurrently to that buffer which is not safe.

This replaces the shared static confstr[Maxconf] buffer with a
pointer that is initially nil and a buffer that is alloced on
demand.

The new confstr pointer (and buffer) is now only updated while
wlock()ed from the new setconfstr() function.

This is now done by mconfig() / mdelctl() just before releasing
the wlock.

Now, rdconf() will check if confstr has been initialized, and
test for it again while wlock()ed; making sure the configuration
is read only once.

Also, rdconf() used to check for a undocumented "fsdev:\n" string
at the beginning of config data tho that was never documented.

This changes mconfig() to ignore that particular signature so
the example from the manpage will work as documented.