Split libuev.c into smaller files to help linker

Split libuev.c into one file per watcher type, mostly to give a better
overview and ease of maintenance, but also to help the linker.

Unfortunately this steals a few bytes when linking a full blown app, but
for apps that, e.g., only need I/O watchers and no timers the resulting
binary will be smaller.  This applies to static binaries that are more
common in embedded systems.

$ size libuev.a libuev.so.1
   text	   data	    bss	    dec	    hex	filename
    999	      0	      0	    999	    3e7	main.o (ex libuev.a)
     90	      0	      0	     90	     5a	io.o (ex libuev.a)
    490	      0	      0	    490	    1ea	timer.o (ex libuev.a)
   4380	    696	      8	   5084	   13dc	libuev.so.1

Also, move uev.h back to top from include/libuev/uev.h since a separate
namespace is not needed after the removal of the OpenBSD queue.h copy.

Signed-off-by: Joachim Nilsson <troglobit@gmail.com>

Makefile

@@ -36,11 +36,11 @@ LIBNAME     = libuev
 prefix     ?= /usr/local
 libdir     ?= $(prefix)/lib
 datadir    ?= $(prefix)/share/doc/$(LIBNAME)
-incdir     ?= $(prefix)/include/$(LIBNAME)
+incdir     ?= $(prefix)/include
 DISTFILES   = README LICENSE test.c
-INCLUDES    = uev.h queue.h
+HEADER      = uev.h
 
-OBJS       := libuev.o
+OBJS       := main.o io.o timer.o
 SRCS       := $(OBJS:.o=.c)
 DEPS       := $(addprefix .,$(SRCS:.c=.d))
 VER         = 1
@@ -77,9 +77,7 @@ install: strip
 	@install $(STATICLIB) $(DESTDIR)$(prefix)/lib/$(STATICLIB)
 	@ln -sf $(SOLIB) $(DESTDIR)$(prefix)/lib/$(SYMLIB)
 	@install -d $(DESTDIR)$(incdir)
-	@for file in $(INCLUDES); do \
-		install -m 0644 include/libuev/$$file $(DESTDIR)$(incdir)/$$file; \
-	done
+	@install -m 0644 $(HEADER) $(DESTDIR)$(incdir)/$(HEADER)
 	@install -d $(DESTDIR)$(datadir)
 	@for file in $(DISTFILES); do \
 		install -m 0644 $$file $(DESTDIR)$(datadir)/$$file; \