Fix issue detected by clang

Simplify apps/ Makefiles by combining common logic into the Makefile fragment Application.mk

Macros PR_BEGIN_EXTERN_C and PR_END_EXTERN_C were not defined in all contexts.  Remove definition and replace with explicit expansion.  From Stefan Kolb

Correct #if to #ifdef when the macro can be undefined

apps/nshlib: Fix error handling in 'mv' command.  On a failure to expand the second path, the memory allocated for the expansion of the first path was not being freed.  From Bruno Herrera.

apps/nshlib: Fix error handling in 'cat' command.  On a failure to allocate memory, a file was not being closed.  From Bruno Herrera.

examples/nettest: Fix a couple of crazy long delays that may make you think something is broken.  In main, add a wait for the server to exit

apps/system/netdb would not build unless CONFIG_NETDB_HOSTFILE was defined because it needed gethostbyname.  Noted by OrbitalFox

apps/examples/usbserial: Can now be run as an NSH builtin-function. Now uses a configurable IO buffer size