L::Util: Introduce "{,l,r}strip" ...

... to replace uses of s/^\s++//; or/and s/\s++$//;.  In void context
they modify their input inplace and in other contexts they copy the
input.  Thus they can replace the "chomp" like:

  while ( my $line = <>) {     while (<>) {
     strip ($line);               strip;
     # do something ...           # do something...
  }                            }

For cases where the original input might be useful, they can also
return a copy:

  while ( my $orig = <>) {
     my $stripped = strip ($orig);
  }

Signed-off-by: Niels Thykier <niels@thykier.net>