ash: use pgetc_eatbnl() in more places

ash: use pgetc_eatbnl() in more places

Part of upstream commit:

    Date:   Thu Mar 8 08:37:11 2018 +0100
    Author: Harald van Dijk <harald@gigawatt.nl>
    parser: use pgetc_eatbnl() in more places

    dash has a pgetc_eatbnl function in parser.c which skips any
    backslash-newline combinations. It's not used everywhere it could be.
    There is also some duplicated backslash-newline handling elsewhere in
    parser.c. Replace most of the calls to pgetc() with calls to
    pgetc_eatbnl() and remove the duplicated backslash-newline handling.

    Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Not adding "readtoken1(pgetc_eatbnl(), DQSYNTAX..." changes, since
readtoken1() handles the "starts with backslash + newline" case itself.

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>