Don't store peeked characters in a char variable

peekNextDigit() returns an int, so it can return -1 in addition to all
256 possible bytes. By putting the result in a signe char, all bytes
over 128 will be interpreted as "no bytes available". Furthermore, it
seems that on SAM "char" is unsigned by default, causing the
"if (c < 0)" line a bit further down to always be false.

Using an int is more appropriate.

A different fix for this issue was suggested in #1399. This fix helps
towards #1728.
Suppress "unused parameter" warnings

A bunch of functions have parameters they do not use, but which cannot
be removed for API compatibility.

In syscalls_sam3.c, there are a lot of these, so this adds an "UNUSED"
macro which adds the "unused" variable attribute if supported (GCC
specific), or is just a noop on other compilers.

In CDC.cpp, there's only three of these variables, so this commit just
forces a dummy evaluation of them to suppress the warnings.

This helps towards #1792.
Remove check that is always false

len is an unsigned variable, so it will never be less than 0.

This helps towards #1792.
Include stdio.h in dtostrf.c

This makes the declaration of sprintf available, so the function is not
implicitely declared, which triggers two compiler warnings.

This helps towards #1792
Fix two signedness warnings

This helps towards #1792
Remove const specifier from channelToTC array in analogWrite on SAM

Members of this array are later passed to functions that accept
non-const pointers. These functions probably don't modify their
arguments, so a better solution would be to update those functions to
accept const pointers. However, they look like third-party code, so that
would require changing the code again on every update. Removing const
here fixes at least the compiler warning for now.

This helps towards #1792.