Remove "extern" from method prototypes

extern is the default behaviour for method prototypes that isn't
declrared as static. Explicitly specify extern in the header files just
adds to the noise for the person reading the header files.

Signed-off-by: Max Sikstrom <msikstrom@op5.com>
Merge branch 'jbop-dev/no_extern'

Signed-off-by: Robin Sonefors <robin.sonefors@op5.com>

Conflicts:
	naemon/common.h
	naemon/workers.h