Build getcap.c only once.

Centralize the definition of the tool name prefix (i.e. "nb") into
bsd.own.mk, as the _TOOL_PREFIX variable.

Roll in fixes to permit cross-compiling from non-NetBSD hosts. This
round has been tested on Solaris/x86 and Linux hosts.

* Add host tools cap_mkdb, ctags, m4, uudecode.
* Protect __RCSID() and __C

Roll in fixes to permit cross-compiling from non-NetBSD hosts. This
round has been tested on Solaris/x86 and Linux hosts.

* Add host tools cap_mkdb, ctags, m4, uudecode.
* Protect __RCSID() and __COPYRIGHT() better.
* Reduce the number of places that need to include "config.h", to keep
sources closer to their "vanilla" versions.
* Add more compat #defines and autoconf-checked functions.

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