troff: support `\~` as an alias for `\ `

[g-branden-robinson]

In groff, Heirloom Doctools troff, and neatroff, the \~ escape sequence produces an adjustable (paddable) but unbreakable space. mandoc, which does not perform adjustment or render to typesetters, supports the escape sequence as a synonym for \, the same as \0.

This extension is of long pedigree: groff has supported it for at least 30 years, Heirloom for 17, mandoc for 13, and neatroff for 6.

Do the same as mandoc to prevent mis-rendering of documents using this escape sequence. Heirloom Doctools troff, a descendant of Documenter's Workbench troff, would be a good place to look for a compatible implementation of the adjustable semantics for this sequence.

[g-branden-robinson]

I tested the patch as follows.

$ printf 'foo\\~bar\n' | 9 troff > unpatched.trout
[change n1.c]
$ printf 'foo\\~bar\n' | 9 troff > patched.trout
$ diff -u unpatched.trout patched.trout 
--- unpatched.trout     2022-08-12 18:13:52.023987607 -0500
+++ patched.trout       2022-08-12 18:14:02.743938198 -0500
@@ -18,7 +18,7 @@
 H720
 V120
 cf
-33o50o50~33b50a44rn120 0
+33o50o50 25b50a44rn120 0
 x trailer
 V7920
 x stop