strbuf.c|h: Add the constant version initialization method of strbuf

[adlternative]

According to
https://public-inbox.org/git/nycvar.QRO.7.76.6.1806210857520.11870@tvgsbejvaqbjf.bet/,
we can create a new way to initialize strbuf with string constant
to save the overhead of dynamically allocated memory.
the marco function STRBUF_INIT_CONST(str).For Example,STRBUF_ININ_DEFAULT("default")
will create a strbuf which buf eqoal to "default",len eqoal to 7 and alloc eqoal to 0.
this constant strbuf will be changed to dynamic allocation when we want to modify the contents of the constant buffer,
the function strbuf_const_to_no_const do this by change buf with xstrdup,
some function like strbuf_grow and strbuf_setlen will use it,
to change the strbuf to a dynamically expand buffer.

Signed-off-by: ZheNing Hu adlternative@gmail.com

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Prefixed commit message must be in lower case: strbuf.c|h: Add the constant version initiazation method of strbuf

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There are issues in commit 85e1e49:
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Prefixed commit message must be in lower case: strbuf.c|h: Add the constant version initialization method of strbuf

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Prefixed commit message must be in lower case: strbuf.c|h: Add the constant version initiazation method of strbuf

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First line of commit message is too long (> 76 columns): strbuf.c:change function strbuf_const_to_no_const with xmalloc instead of use xstrdup.

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There are issues in commit 85e1e49:
The first line must be separated from the rest by an empty line
Prefixed commit message must be in lower case: strbuf.c|h: Add the constant version initialization method of strbuf

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There are issues in commit e1bce64:
The first line must be separated from the rest by an empty line
Prefixed commit message must be in lower case: strbuf.c|h: Add the constant version initiazation method of strbuf

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There is an issue in commit 25ded82:
First line of commit message is too long (> 76 columns): strbuf.c:change function strbuf_const_to_no_const with xmalloc instead of use xstrdup.