DROPPRIVS_USER in manpage is undefined

[cody-somerville]

In the ngrep manpage, in reference to the -R command line option, it reads

Do not try to drop privileges to the DROPPRIVS_USER

However, there is no other use of DROPPRIVS_USER in the man page and it is left undefined.

Written as is, due to conventions commonly used in some manpages and command line help text, a user may mistakenly assume that DROPPRIVS_USER can be specified at runtime such as via an argument to the option, environmental variable, or key in a configuration file despite it being statically set at compile time. Considering the potential unforeseen security implications of a user acting on that assumption (or time spent investigating), it may be prudent to update the manpage to remove the opportunity for confusion.

Potential options could be to remove the reference to DROPPRIVS_USER all together or instead elaborate that DROPPRIVS_USER is a compile time option and optionally mention the default value or insert the value used into the manpage at build time.