Including string.h in base.h causes any file that includes a BoringSSL
header to include string.h. Generally this wouldn't be a problem,
although string.h might slow down the compile if it wasn't otherwise
needed. However, it also causes problems for ipsec-tools in Android
because OpenSSL didn't have this behaviour.

This change removes string.h from base.h and, instead, adds it to each
.c file that requires it.

Change-Id: I5968e50b0e230fd3adf9b72dd2836e6f52d6fb37
Reviewed-on: https://boringssl-review.googlesource.com/3200
Reviewed-by: David Benjamin <davidben@chromium.org>
Reviewed-by: Adam Langley <agl@google.com>

Before it was possible to pass a NULL-terminated C-string to the PBKDF2
functions, and indicate the parameter was a C-string by passing a length
of -1.

This is not relied on anywhere in the BoringSSL code, and the API contract is
possible to misuse as it is not the common way of doing things.

(A problem would arise when passing in a large unsigned length that
subsequently gets interpreted as -1).

Change-Id: Ifbd31ff76e183fa74e9fa346908daf4bfb8fc3da
Reviewed-on: https://boringssl-review.googlesource.com/2953
Reviewed-by: David Benjamin <davidben@chromium.org>
Reviewed-by: Adam Langley <agl@google.com>

Initial fork from f2d678e6e8 (1.0.2 beta).

(This change contains substantial changes from the original and
effectively starts a new history.)