…nt to EthernetClient() anyway

constructor m_client(MAX_SOCK_NUM) is not portable (fails to compile with UIPEthernet)

Trim down the size of a couple of variables as they didn't need to be as
large, clarify the type of some others, and change port from int to
uint16_t to match EthernetServer.

Change the definition of the P() macro under the AVR architecture to store
the strings in sections named after the variables.  This allows unused
strings to be removed by the linker, saving space in the flash memory.

Also change the variable name for all but one of the instances of failMsg
to give the maximum benefit from the above.

Modify the functions which send headers to re-use one string containing
the server name instead of including it inline in all of the different
headers.

Flash memory saving optimisations

Add macro for configuring maximum number of commands that can be defined.
This macro replaces hardcoded "8" number.

Add WEBDUINO_COMMANDS_COUNT macro

The latest 1.5.7 beta release of Arduino includes an updated toolchain
which now issues warnings about narrowing of ints to chars inside curly
braces becoming ill-formed in C++11.  Explicitly cast the values to char
to prevent this.

Fix compiler warnings about narrowing conversion