fix broken shgetc limiter logic (wasn't working)

use macros instead of inline functions in shgetc.h

at -Os optimization level, gcc refuses to inline these functions even
though the inlined code would roughly the same size as the function
call, and much faster. the easy solution is to make them into macros.

add "scan helper getc" and rework strtod, etc. to use it

the immediate benefit is a significant debloating of the float parsing
code by moving the responsibility for keeping track of the number of
characters read to a different module.

by linking shgetc with the stdio buffer logic, counting logic is
defered to buffer refill time, keeping the calls to shgetc fast and
light.

in the future, shgetc will also be useful for integrating the new
float code with scanf, which needs to not only count the characters
consumed, but also limit the number of characters read based on field
width specifiers.

shgetc may also become a useful tool for simplifying the integer
parsing code.