Add 4 functions missing from perlfunc.pod's 'Perl Functions by Category'.

Add 4 functions missing from perlfunc.pod's 'Perl Functions by Category'.

C<readline> added to 'Input and output functions'
Documentation on readline was added by commit 8490252049bf42d3 in Aug 1997.
That commit also added readpipe without adding it to a category group, an
omission that was fixed by commit 4319b00c03e6a517 in Nov 2011.

C<sysseek> added to 'Functions for fixed-length data or records'
Documentation on sysseek was added by commit 137443ea0a858c43 in Apr 1997,
which implemented sysseek.

C<prototype> added to both 'Keywords related to the control flow of your Perl
program' and 'Miscellaneous functions', as Pod::Functions places it in both.
Commit da0045b73af6f504 in Jul 1996, included 'Add documentation for new
"prototype" operator', but only added the main documentation, not an entry
in a category group.

C<lock> added to 'Miscellaneous functions'.
Documentation on lock was added by commit 19799a22062ef658 in May 1999.