cmdgen: rescue test suite from bit rot.

cmdgen.c has contained code for ages to build a test main() if you
compile with -DTEST_CMDGEN. But it's painful to do so manually, since
you've still got to link in all the same supporting objects, and also
nobody can have actually done that for a while because the stub test
code hasn't been kept up to date with changes in the internal APIs
(specifically prompt_t).

Now we have the ability to include our test programs in Recipe as [UT]
or [XT] so as to leave them out of 'make install', that seems like a
useful thing to do with cmdgen's test suite. So here's a Recipe change
that builds it as 'cgtest', plus fixes for compiler warnings and bit
rot. Pleasantly, the test suite still _passes_ after those are fixed.

Recipe

@@ -311,10 +311,12 @@ puttytel : [X] GTKTERM uxmisc misc ldisc settings uxsel U_BE_NOSSH
 plink    : [U] uxplink uxcons NONSSH UXSSH U_BE_ALL logging UXMISC uxsignal
          + ux_x11 noterm
 
-puttygen : [U] cmdgen sshrsag sshdssg sshprime sshdes sshbn sshmd5 version
+PUTTYGEN_UNIX = sshrsag sshdssg sshprime sshdes sshbn sshmd5 version
          + sshrand uxnoise sshsha misc sshrsa sshdss uxcons uxstore uxmisc
          + sshpubk sshaes sshsh256 sshsh512 IMPORT puttygen.res time tree234
          + uxgen notiming conf sshecc sshecdsag
+puttygen : [U] cmdgen PUTTYGEN_UNIX
+cgtest   : [UT] cgtest PUTTYGEN_UNIX
 
 pscp     : [U] pscp uxsftp uxcons UXSSH BE_SSH SFTP wildcard UXMISC
 psftp    : [U] psftp uxsftp uxcons UXSSH BE_SSH SFTP wildcard UXMISC