fix gratuitous undefined behavior in strptime

accessing an object of type const char *restrict as if it had type
char * is not defined.

implement %y and %C specifiers in strptime

fix unhandled cases in strptime

%C, %U, %W, and %y handling were completely missing; %C wrongly
fell-through to unrelated cases, and the rest returned failure. for
now, they all parse numbers in the proper forms and range-check the
values, but they do not store the value anywhere.

it's not clear to me whether, as "derived" fields, %U and %W should
produce any result. they certainly cannot produce a result unless the
year and weekday are also converted, but in this case it might be
desirable for them to do so. clarification is needed on the intended
behavior of strptime in cases like this.

%C and %y have well-defined behavior as long as they are used together
(and %y is defined by itself but may change in the future).
implementing them (including their correct interaction) is left as a
later change to be made.

finally, strptime now rejects unknown/invalid format characters
instead of ignoring them.

include cleanups: remove unused headers and add feature test macros

use restrict everywhere it's required by c99 and/or posix 2008

to deal with the fact that the public headers may be used with pre-c99
compilers, __restrict is used in place of restrict, and defined
appropriately for any supported compiler. we also avoid the form
[restrict] since older versions of gcc rejected it due to a bug in the
original c99 standard, and instead use the form *restrict.

fix bugs in strptime handling of string day/month names, literals

strptime: fix use of uninitialized dest field in converting integer

partially working strptime

it's missing at least:
- derived fields
- week numbers
- short year (without century) support
- locale modifiers

initial check-in, version 0.5.0