Remove unused variable from ethernet socket utils

Remove the variable 'status' as this had a value assigned (from readSnSR),
but it was never used.
Cast empty string to char* to fix compiler warning

Stream::find(char *target) passes an empty terminator string to
Stream::findUntil(char *target, char *terminator) which caused a compiler
warning with the updated toolchain, so cast it to a char*.
Add availableForWrite() to HardwareSerial
Merge pull request #2193 from PaulStoffregen/ide-1.5.x

Add availableForWrite() to HardwareSerial
Merge pull request #2190 from ribbons/compiler-warnings

Fix two compiler warnings generated by updated toolchain
Merge pull request #2194 from Fede85/ide-1.5.x_lib-description

Ide 1.5.x lib description
fixed some broken links in library.properties files