doc/nethack.6: Revise "ENVIRONMENT" section

Explain meaning of stacked paragraph tags naming environment variables.

Drop call of deprecated man(7) `DT` macro and invocation of `ta` request
to set tab stops to attempt table-like layout.  (The latter made the
former nilpotent anyway.)  Replace this material with calls of tagged
paragraphing macro `TP` and groff man(7)'s `TQ` extension for setting
multiple tags with a paragraph.  The result takes up more screen lines,
but renders well with more formatters.

Set indentation of tagged paragraph using a constant numeric expression
to accommodate pseudo-roff formatters that don't implement arithmetic
evaluation.  (Some of these ignore _any_ indentation, regardless.)

Slightly recast descriptions of environment variables.

Set file name literals in italics.

Protect file and environment variable names from hyphenation.

Fixes:

an.tmac:doc/nethack.6:608: style: use of deprecated macro: .DT

doc/nethack.6

@@ -605,34 +605,70 @@ In a perfect world,
 .I paniclog
 would remain empty.
 .SH ENVIRONMENT
-.DT
+In some cases,
-.ta \w'NETHACKDIR or HACKDIR\ \ \ 'u
+.I NetHack
-USER or LOGNAME	Your login name.
+attempts to access multiple environment variables in sequence
-.br
+(via
-HOME		Your home directory.
+.IR getenv (3)
-.br
+or similar)
-SHELL		Your shell.
+to determine information.
-.br
+.TP 16n \" "NETHACKOPTIONS" + 2n
-TERM		The type of your terminal.
+.I USER
-.br
+.TQ
-HACKPAGER or PAGER	Replacement for default pager.
+.I
-.br
+LOGNAME
-MAIL	Mailbox file.
+Your login name.
-.br
+.TP
-MAILREADER	Replacement for default reader
+.I HOME
-.br
+Your home directory.
-\ 	(probably /bin/mail or /usr/ucb/mail).
+.TP
-.br
+.I SHELL
-NETHACKDIR or HACKDIR	Playground.
+Your login shell
-.br
+(not necessarily the one running when you started
-NETHACKOPTIONS	String predefining several NetHack options.
+.IR NetHack ).
+.TP
+.I TERM
+Your terminal type.
+.TP
+.I HACKPAGER
+.TQ
+.I PAGER
+Replacement for default pager.
+.TP
+.I MAIL
+Mailbox file.
+.TP
+.I MAILREADER
+Replacement for default reader
+(probably
+.I /bin/\*:mail
+or
+.IR /usr/\*:ucb/\*:mail ).
+.TP
+.I NETHACKDIR
+.TQ
+.I HACKDIR
+Playground.
+.TP
+.I NETHACKOPTIONS
+Predefined
+.I NetHack
+options.
 .PP
-If the same option is specified in both NETHACKOPTIONS and .nethackrc,
+If the same option is specified in both
-the value assigned in NETHACKOPTIONS takes precedence.
+.I \%NETHACKOPTIONS
+and
+.IR \%.nethackrc ,
+the value assigned in
+.I \%NETHACKOPTIONS
+takes precedence.
 .PP
-SHOPTYPE and SPLEVTYPE can be used in debugging (wizard) mode.
+.I SHOPTYPE
-.br
+and
-DEBUGFILES can be used if the program was built with 'DEBUG' enabled.
+.I \%SPLEVTYPE
+can be used in debugging (wizard) mode.
+.I \%DEBUGFILES
+can be used if the program was built with 'DEBUG' enabled.
 .SH "SEE ALSO"
 .PP
 recover(6)