o Replace two while {} do loops with more appropriate do {} while loops. This

o Replace two while {} do loops with more appropriate do {} while loops. This
  doesn't change functionality, but makes code more logical.

Obtained from:	DrafonFlyBSD

o Use VOP_GETATTR() to obtain actual size of file and parse no more than that.
  Previously, we parsed MAXSHELLCMDLEN characters regardless of the actual file
  size. This makes the following working:

$ printf '#!/bin/echo' > /tmp/test.sh
$ chmod 755 /tmp/test.sh
$ /tmp/test.sh

Previously, attempts to execve() that shell script has been failing with bogus
ENAMETOOLONG.

PR:		kern/64196
Submitted by:	Magnus B.ckstr.m <b@etek.chalmers.se>