More -Wmissing-variable-declarations fixes.

In addition to adding missing `static' keywords:

• bin/dd: Pull in `extern.h' to guarantee consistency with source file.
• libexec/rpc.rusersd: Move shared globals into an extern.h.
• libexec/talkd: Move debug' and hostname' into extern.h.
• usr.bin/cksum: Put counters in extern.h, as they are used by ckdist/mtree.
• usr.bin/m4: Move `end_result' into extern.h.
• usr.sbin/services_mkdb: Move shared globals into an extern.h.