cmSystemTools: Add StringToInt helper

Convert a string to a signed integer and reject any extra input.
file: Add LOCK subcommand to do file and directory locking

Provide options to fail without blocking or to block up to a timeout.
Provide options to specify the scope containing the lock so it can be
released automatically at the end of a function, file, or process.

Extend the RunCMake.file test with cases covering the file(LOCK) command
usage and error cases.
Help: Add notes for topic 'file-LOCK-command'
Merge topic 'file-LOCK-command' into next

ed00a808 Help: Add notes for topic 'file-LOCK-command'
fca624ca file: Add LOCK subcommand to do file and directory locking
5121f118 cmSystemTools: Add StringToInt helper