Fix hal/drivers/*.comp build

The `$(COMP_DRIVERS)` variable was left out of `obj-m`, so these comps
have not been built for a long time.  Re-enabling revealed a number of
build problems:

- The pluto driver build was only partially disabled for userland threads
  - legacy project completely disables for userland
  - enabling userland threads build reveals more problems:
    - `read()`/`write()` name conflict with functions in `unistd.h`
    - `llabs` type definition conflict
- The `pcl720` and `serport` comps needed header updates for `USER_PCI`
- Broken `msgcomponents` comp build recipes
  - `hal/components` and `machinetalk/msgcomponents` don't fit into
    one pattern
  - man pages not built

This patch straightens the basic build problems out, but these fixes
are untested.  Fixes #384