Allow the #! command line to be up to PAGE_SIZE long, rather then

Allow the #! command line to be up to PAGE_SIZE long, rather then
artificially limiting it to MAXSHELLCMDLEN.  The interpreter name is
still limited to MAXSHELLCMDLEN.

A \0 is now considered a line terminator.

Correctly report the ENAMETOOLONG case.

Note that FreeBSD (and hence DragonFly) break the #! line into multiple
argv arguments, which is not what any other UNIX does.  This is likely going
to be changed in the future but not as of this commit.

Inspired-by: Maxim Sobolev <sobomax@FreeBSD.org> / similar FreeBSD work,
		and Garance A Drosihn <drosih@rpi.edu>