Move server API concept to base framework

This commit introduces the new `Component` interface in the form of the
headers base/component.h and base/entrypoint.h. The os/server.h API
has become merely a compatibilty wrapper and will eventually be removed.
The same holds true for os/signal_rpc_dispatcher.h. The mechanism has
moved to base/signal.h and is now called 'Signal_handler'.

Since the patch shuffles headers around, please do a 'make clean' in the
build directory.

Issue #1832

repos/base/lib/import/import-ld.mk

@@ -0,0 +1,6 @@
+#
+# All dynamic executables must be linked to the component entry-point library
+# (a trampoline for component startup from ldso), so, enforce the library
+# dependency here.
+#
+LIBS += component_entry_point

repos/base/lib/mk/component_entry_point.mk

@@ -0,0 +1,7 @@
+#
+# Component entry point (a trampoline for component startup from ldso)
+#
+
+SRC_CC = component_entry_point.cc component_construct.cc
+
+vpath %.cc $(REP_DIR)/src/lib/startup

repos/base/lib/mk/ldso.inc

@@ -24,6 +24,8 @@ ENTRY_POINT = _start
 LD_OPT     += -T$(DIR)/linker.ld
 endif
 
+include $(REP_DIR)/lib/import/import-ld.mk
+
 vpath %.cc $(DIR)
 
 # vi:ft=make

repos/base/lib/mk/startup.inc

@@ -1,5 +1,5 @@
 SRC_S       += crt0.s
-SRC_CC      += _main.cc init_main_thread.cc
+SRC_CC      += _main.cc init_main_thread.cc component_construct.cc
 REP_INC_DIR += src/include
 LIBS        += syscall