[MINOR] buffers: add peekchar and peekline functions for stream interfaces

[MINOR] buffers: add peekchar and peekline functions for stream interfaces

The buffer_si_peekline() function is sort of a fgets() to be used from a
stream interface. It returns a complete line whenever possible, and does
not update the buffer's pointer, so that the reader is free to consume
what it wants to.

buffer_si_peekchar() only returns one character, and also needs a call
to buffer_skip() once the character is definitely consumed.