doc: man: consistently italicize --option parameters

doc: man: consistently italicize --option parameters

This changes a few pages, but the changes in tail.1
concisely illustrate the resulting man page changes:

  $ diff -r man.orig/tail.1 man/tail.1
  < \fB\-c\fR, \fB\-\-bytes\fR=\fI\,[\/\fR+]NUM
  > \fB\-c\fR, \fB\-\-bytes\fR=\fI\,[+]NUM\/\fR

  < \fB\-f\fR, \fB\-\-follow[=\fR{name|descriptor}]
  > \fB\-f\fR, \fB\-\-follow\fR[=\fI\,{name|descriptor}\/\fR]

  < \fB\-n\fR, \fB\-\-lines\fR=\fI\,[\/\fR+]NUM
  > \fB\-n\fR, \fB\-\-lines\fR=\fI\,[+]NUM\/\fR

* man/help2man: Relax the option match so more --option
variations are supported, and passed through to convert_option().
Specifically more variations after '=' are now supported.
Also split and document the regular expression.
Reported at https://github.com/coreutils/coreutils/issues/84