Change the undefined values of LOGICAL headers to -12345

Follow the definition of SacHeader.h in `inc'
Reimplent of SAC format read function

- change length of character strings from 8 to 9 to deal with
  the null terminator in C;
- length of kevnm are change to 18;
- a new struct with only character strings are defined. It is used
  while read files from disk and write files to disk.
  I am not sure it is necessary so it may be removed in the future!
- additional macros related to SAC headers are defined. Not sure
  all of them are necessary.
- reimplent ReadSacHead and ReadSac
  - map_chdr_in to map Fortran strings to C strings
  - rsachead is used to read header, map strings and swap bytes
- newhdr to create a new SAC header is written and commented.