Added bstring, a slightly friendlier C string

The bstring type is a very simple struct containing only a string and its
length.  The BSTR macro in libbill.h allows the programmer to extract the
C-string from within a bstring.
Added definitions for bstring.
Added the restrict keyword to appropriate parts of libbill.c
Added the restrict keyword to t_f_p prototype
Added the restrict keyword to text_file_read
Removed invalid use of `restrict` keyword
Removed another invalid use of `restrict`
Removed yet another invalid use of `restrict`
Removed a compiler warning wrt an unused variable
Removed an unused-variable warning
Added bstring to build system
Removed invalid use of `restrict`, fixed compiler warnings
Fixed compilation errors & warnings